1. Field of the Invention
The present invention relates to a technique for improvement in image quality of an image signal in a solid state image sensor used in a video camera, a digital still camera and the like.
2. Description of the Related Art
In digital cameras, video cameras and the like, a CCD image sensor, a CMOS image sensor or the like is used as a solid state image sensor.
In the meantime, it is known that when a high luminance object exists on a CCD image sensor, a CMOS image sensor or the like, as a signal level of its entire row or column is fluctuated, a vertical or horizontal line image appears. This phenomenon is widely known as a smear phenomenon in the CCD image sensors, and a similar phenomenon also occurs in the CMOS image sensors.
In the CMOS image sensor, this phenomenon occurs due to wiring layout of power source, the ground and the like. Generally, the power source and the ground are wired in a horizontal direction. For example, when a large amount of current flows through a high luminance object in a screen, as a result, the power sources of pixels on the left and right sides of the object are fluctuated, thereby a horizontal line image appears due to the fluctuation of signal level of the entire same line or the subsequent lines. This phenomenon remarkably occurs particularly when an image sensor internal circuit is saturated by a high luminance object, since signal output cannot be sufficiently controlled due to the circuit saturation. Such a problem in the circuit cannot be solved without difficulty.
FIGS. 12A and 12B show the above situation. FIG. 12A show signals from a CMOS image sensor in correspondence with a pixel layout.
Generally, the solid state image sensor has an optical black area shielded from light not to react to light so as to obtain a signal level reference signal (reference signal for black level), and an effective pixel signal is subjected to arithmetic processing on the basis of the level obtained with the optical black area. This pixel is referred to as an “OB pixel (Optical Black pixel)”.
In FIG. 12A, areas denoted by “HOB” and “VOB” indicate signals from the above-described optical black pixels. Further, an area denoted by “effective pixel” indicates signals from a pixel area to actually receive light.
FIG. 12A shows a status where high luminance light “P” is received when a peripheral luminance level is low. In the left and right pixel outputs “L” and “R” in the horizontal direction, level fluctuation causes a horizontal line image. As the peripheral luminance is low, ideal output levels of other portions than the portion receiving the high luminance light “P” are the same as those of the HOB and VOB areas.
FIG. 12B shows the image output level on a line A-A in FIG. 12A. The “HOB level” is an average level of areas where the level fluctuation is not caused in the HOB area, and generally this level corresponds to a reference signal for black level. On the other hand, the level of a portion receiving the high luminance light “P” is very high and its output causes circuit saturation. Further, the levels of left and right pixel outputs “L” and “R” are lower than the HOB level and level fluctuation occurs.
FIGS. 13A and 13B show another example where high luminance light exists in the screen. The difference from FIGS. 12A and 12B is that the pixel output “L” on the left side of the high luminance light “P” is lower than the HOB level while the pixel output “R” on the right side of the high luminance light is higher than the HOB level, thus level fluctuation occur. This phenomenon is empirically confirmed. It is considered that the above situation occurs due to the readout order of pixel signals, level fluctuation convergence time and the like, in addition to the wiring layout of the power source and the ground.
For example, in a general CMOS image sensor, signals from 1-line pixels in the horizontal direction are transferred at once to a column readout circuit, then the signals transferred to the column readout circuit are sequentially output from a left pixel in the screen through a final stage common readout circuit. At this time, when the level change occurs not in pixels of the image sensor or the column readout circuit but in the final stage readout circuit, the right side “R” is read immediately after the occurrence of level fluctuation caused by the high luminance light “P”, and the left side “L” in the next line is read. It is conceivable that the direction and amount of level fluctuation is changed due to the difference in readout time.
Next, correction of the horizontal line image due to the above-described level fluctuation will be discussed.
Regarding the smear phenomenon in the CCD image sensor or the like, as a similar smear occurs in optical black pixels, Japanese Patent Laid-Open Nos. 07-67038 and 2001-24943 disclose correction on outputs of effective pixels based on an average value of signal values of optical black pixels in the same column.
Especially in Japanese Patent Laid-Open No. 2001-24943, the content of correction processing in smear correcting means is changed in correspondence with the level of an effective image signal. More particularly, when the level of the effective image signal is equal to a predetermined signal saturation level, the smear correction is not performed, thereby the occurrence of darkened image degraded portion due to overcorrection of the saturated portion by the smear correction can be prevented.
In the CMOS image sensor, when level fluctuation occurs on the left and right sides of the high luminance light “P” as shown in FIGS. 12A and 12B, the correction can be performed as disclosed in Japanese Patent Laid-Open Nos. 07-67038 and 2001-24943. However, as shown in FIGS. 13A and 13B, when the left pixel output “L” of the high luminance light “P” is lower than the HOB level while the right pixel output “R” is higher than the HOB level, thus level changes occur, the correction cannot be performed in a similar manner. More particularly, FIGS. 14A and 14B show the result of correction on the outputs of the effective portions based on an average value of HOB pixel signals in the same line. That is, the left pixel output “L” close to the HOB can be corrected, while regarding the right pixel output “R” far from the HOB, as the direction and amount of the level fluctuation are different, it cannot be corrected, and further, the level fluctuation might be increased.
Similarly, in other image sensors such as a CCD image sensor than the CMOS image sensor, when the levels in the left and right or upper and lower sides of the high luminance light “P” are not even but level fluctuation occur, correction cannot be performed.